The burning of fuels, however it is accomplished in burners as now known in the art of fuel burning, is productive of oxides of nitrogen (NOx) in normal operation. Such oxides of nitrogen as are produced, in combination with olefinic hydrocarbons, which may be present in the atmosphere, constitute a source of smog.
Smog, while not necessarily lethal is recognized universally as potentially damaging to animal tissue. Consequently, severe limitations on the Nox content of stack gases vented to the atmosphere, as a result of fuels burning, have been imposed by various governmental authorities and agencies. Emission of olefinic hydrocarbons is also subject to limitation, but is a matter separate from the invention of this application.
Research has shown clearly that when cooled combustion gases are recirculated for mixture with air for combustion prior to entry of the air-flue gas mixture to the combustion reaction zone in a burner, the result is reduced NOx concentration in the combustion gases following the combustion. Since the flue gases contain CO.sub.2 and water vapor this desirable result is probably due to the following high temperature chemistries: EQU CO.sub.2 + CH.sub.4 = 2 CO + 2 H.sub.2 EQU H.sub.2 O + CH.sub.4 = CO + 3H.sub.2
These reactions occur within the body of the flame in regions where oxygen is not present but NOx is present. The NOx promptly reacts with both carbon monoxide and hydrogen as sources of oxygen, with formation of carbon dioxide and water and to produce lowered NOx in the flue gases emitted from the venting device to the atmosphere. This condition is where combustion is complete prior to flue gas recirculation and cooling.
Further research has shown with equal clarity, that if the combustion gases are from incomplete combustion and are also cooled prior to mixture with the air for combustion, the presence in them of both carbon monoxide and hydrogen resulting from incomplete combustion, as well as carbon dioxide and water vapor further reduces NOx emission to levels of concentration in flue gases significantly below the emission state due to the carbon dioxide, water vapor and methane reaction alone. Research has shown then that for minimum NOx emission in combustion gases there should be recirculation of the combustion gases as discussed. The recirculated combustion gases require cooling and they should be from an area where combustion is not complete.
It is known in the art to withdraw stack gases which have been cooled by heat utilization, and to deliver the cooled stack combustion gases to selected burner areas for NOx reduction. Thus the simple recirculation of combustion gases is known in the art. However, in this invention, the recirculation gases are drawn from an area where the combustion is not complete. Also, cooling of the gases is a characteristic of the burner structure of this invention. Furthermore, no external ducts or vents are required for combustion as recycling.